The correct wiring together of the developing nervous system requires that out-growing neuronal processes navigate accurately through an extremely complex environment. We are interested in identifying some of the environmental cues which guide process outgrowth. Recently, we have focused our attention on specific cues which inhibit the extension of selected processes. Our objective is to identify molecules in the central nervous system of vertebrates which specifically inhibit the motility of the growth cones at the tips of processes. Our aim is to (1) devise and perfect bioassays in culture for motility inhibiting activities, (2) use biochemical techniques in tandom with these assays to purify inhibiting activities, and (3) raise monoclonal antibodies against the purified molecules, and (4) use the monoclonal antibodies to study the distributions and functions of these activities in the embryonic central nervous system. Once we have developed immunological probes for these cues and have some understanding of their roles in normal development, it could be profitable to search for them in mature nervous systems. If present, they could help to explain why regeneration is so difficult to obtain in the central nervous systems of higher vertebrates.